Device using self-organizable polymer

ABSTRACT

A thin film transistor wherein the semiconductor layer is prepared by a process including:  
     creating a composition comprising a liquid and a self-organizable polymer at least partially dissolved in the liquid, resulting in dissolved polymer molecules;  
     reducing the solubility of the dissolved polymer molecules to induce formation of structurally ordered polymer aggregates in the composition;  
     depositing a layer of the composition including the structurally ordered polymer aggregates; and  
     drying at least partially the layer resulting in the structurally ordered semiconductor layer, wherein the structural order of the semiconductor layer increases the charge transport capability of the semiconductor layer.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional application of co-pending U.S.application Ser. No. 10/273,901 (filing date Oct. 17, 2002) from whichpriority is claimed, the disclosure of which is totally incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] Polymer thin film transistors have potential applications as keyelements of integrated circuits for optoelectronic devices based onsolution processing to lower manufacturing cost. Polymer thin filmtransistors generally have lower electrical performance such as lowermobility than do their silicon counterparts such as crystalline siliconor polysilicon thin film transistors. Nevertheless, the electricalperformance levels of polymer thin film transistors may be sufficientfor many applications, particularly large-area devices such as activematrix liquid crystal display, electronic paper, and certain low-endmicroelectronics such as smart cards, radio frequency identificationtags, and the like, where high switching speeds may not be required. Forthese applications, low cost is particularly important. One mainapproach to obtaining higher charge carrier mobility for polymer thinfilm transistors is to achieve higher degrees of structural order in theactive semiconducting polymer layer. PCT WO 00/79617 A1 discloses thealignment of the polymer chains parallel to each other by bringing thepolymer into a liquid-crystalline phase. There is a need, which thepresent invention addresses, for new techniques applicable to thefabrication of electronic devices to induce increased structural orderin semiconducting polymers.

[0003] The following documents also may be relevant:

[0004] F. Brustolin et al., “Highly Ordered Structures of AmphiphilicPolythiophenes in Aqueous Media,” Macromolecules, Vol. 35, pp. 1054-1059(published on web Jan. 3, 2002).

[0005] G. Dufresne et al., “Thermochromic and Solvatochromic ConjugatedPolymers by Design,” Macromolecules, Vol. 33, pp. 8252-8257 (publishedon web Sep. 30, 2000).

[0006] M. Leclerc, “Optical and Electrochemical Transducers Based onFunctionalized Conjugated Polymers, Adv. Mater., Vol. 11, No. 18, pp.1491-1498 (1999).

[0007] Yiliang Wu et al., U.S. Ser. No. 10/273,896 (attorney docketnumber D/A2395), filed Oct. 17, 2002.

SUMMARY OF THE DISCLOSURE

[0008] The present invention is accomplished in embodiments by providinga process comprising:

[0009] creating a composition comprising a liquid and a self-organizablepolymer at least partially dissolved in the liquid, resulting indissolved polymer molecules;

[0010] reducing the solubility of the dissolved polymer molecules toinduce formation of structurally ordered polymer aggregates in thecomposition;

[0011] depositing a layer of the composition including the structurallyordered polymer aggregates; and

[0012] drying at least partially the layer to result in a structurallyordered layer, wherein the structurally ordered layer is part of anelectronic device and the structurally ordered layer exhibits increasedcharge transport capability.

[0013] There are also provided in embodiments a process comprising:

[0014] creating a composition comprising a liquid and a self-organizablepolymer at least partially dissolved in the liquid, resulting indissolved polymer molecules, wherein the polymer is polythiophene;

[0015] reducing the solubility of the dissolved polymer molecules toinduce formation of structurally ordered polymer aggregates in thecomposition, wherein the reducing the solubility of the dissolvedpolymer molecules is accomplished by adding a different liquid that isless capable of dissolving the polymer than the liquid;

[0016] depositing a layer of the composition including the structurallyordered polymer aggregates; and

[0017] drying at least partially the layer to result in a structurallyordered layer, wherein the structurally ordered layer is part of anelectronic device and the structurally ordered layer exhibits increasedcharge transport capability.

[0018] There are further provided in embodiments a thin film transistorcomprising:

[0019] an insulating layer;

[0020] a gate electrode;

[0021] a structurally ordered semiconductor layer;

[0022] a source electrode; and

[0023] a drain electrode,

[0024] wherein the insulating layer, the gate electrode, thesemiconductor layer, the source electrode, and the drain electrode arein any sequence as long as the gate electrode and the semiconductorlayer both contact the insulating layer, and the source electrode andthe drain electrode both contact the semiconductor layer,

[0025] wherein the semiconductor layer is prepared by a processcomprising:

[0026] creating a composition comprising a liquid and a self-organizablepolymer at least partially dissolved in the liquid, resulting indissolved polymer molecules;

[0027] reducing the solubility of the dissolved polymer molecules toinduce formation of structurally ordered polymer aggregates in thecomposition;

[0028] depositing a layer of the composition including the structurallyordered polymer aggregates; and

[0029] drying at least partially the layer resulting in the structurallyordered semiconductor layer, wherein the structural order of thesemiconductor layer increases the charge transport capability of thesemiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Other aspects of the present invention will become apparent asthe following description proceeds and upon reference to the Figureswhich represent illustrative embodiments:

[0031]FIG. 1 represents a first embodiment of a thin film transistormade using the present process;

[0032]FIG. 2 represents a second embodiment of a thin film transistormade using the present process;

[0033]FIG. 3 represents a third embodiment of a thin film transistormade using the present process; and

[0034]FIG. 4 represents a fourth embodiment of a thin film transistormade using the present process.

[0035] Unless otherwise noted, the same reference numeral in differentFigures refers to the same or similar feature.

DETAILED DESCRIPTION

[0036] Any suitable self-organizable polymer which is capable ofincreased structural order in response to reduced solubility of thepolymer in a selected liquid may be used in the present invention.Molecular self-organization refers to the ability of molecules toorganize themselves into a higher molecular and intermolecularstructural order in response to a stimulus such as solvency in a liquid.In embodiments, one, two, three, or more self-organizable polymers maybe used. The self-organizable polymer may or may not precipitate in theliquid. In embodiments of the present invention, there is absent anygelling of the composition comprised of the self-organizable polymer andthe liquid.

[0037] The polymer may be considered to exhibit poor solubility in aliquid when the concentration of the polymer in a saturated solution inthat liquid is not high enough to make a thin polymer layer that isuseful for the intended applications by common deposition techniques.Generally, when the concentration of the polymer in a particular liquidis below about 0.1 percent by weight, its solubility in that liquid isdeemed to be poor. Even though the polymer may exhibit low solubility ina liquid at room temperature, its solubility can generally be increasedby heating above room temperature.

[0038] When the concentration is higher than about 0.2 percent byweight, the polymer is considered to exhibit reasonable solubility as auseful thin polymer layer may be fabricated from this solution usingcommon deposition processes.

[0039] The phrase “room temperature” refers to a temperature rangingfrom about 22 to about 25 degrees C.

[0040] In embodiments, the self-organizable polymer may be a conjugatedpolymer such as for instance polythiophenes, polyphenylenes,polyphenylene derivatives, polyfluorenes, fluorene copolymers, andladder polymers.

[0041] Exemplary polythiophenes include for instance the following:

[0042] where n is from about 5 to about 5,000. Suitable polythiophenesare disclosed in U.S. Ser. No. 10/042,356 (Attorney Docket No. D/A1334),U.S. Ser. No. 10/042,358 (Attorney Docket No. D/A1332), and U.S. Ser.No. 10/042,342 (Attorney Docket No. D/A1333), the disclosures of whichare totally incorporated herein by reference.

[0043] Exemplary polyphenylenes and polyphenylene derivatives includefor instance the following:

[0044] where n is from about 5 to about 5,000; R₁ and R₂ may be the sameor different from each other and are a side chain comprised of, forexample, alkyl derivatives such as alkoxyalkyl and siloxy-subsitutedalkyl, perhaloalkyl such as perfluoroalkyl, polyether such asoligoethylene oxide, polysiloxy derivatives, and the like, containingfrom about 4 to about 50 atoms. Exemplary polyphenylenes, andpolyphenylene derivatives such as poly(phenylene vinylene)s andpoly(phenylene ethynylene)s, are disclosed in “The electroluminescenceof organic materials” by U. Mitschke and P. Bäuerle, J. Mater. Chem.(2000), Vol. 10, pp. 1471-1507, and “Light-Emitting Characteristics ofConjugated Polymers” by H. S. Shim and J. I. Jin, Advance in PolymerScience (2002), Vol. 158, pp. 193-243, the disclosures of which aretotally incorporated herein by reference.

[0045] Exemplary polyfluorenes and fluorene copolymers include forinstance the following:

[0046] where n is from about 5 to about 5,000; R₁ and R₂ may be the sameor different from each other and are hydrogen or a side chain comprisedof, for example, alkyl derivatives such as alkoxyalkyl andsiloxy-subsituted alkyl, perhaloalkyl such as perfluoroalkyl, polyethersuch as oligoethylene oxide, polysiloxy derivatives, and the like,containing from about 4 to 50 atoms. Exemplary polyfluorenes andfluorene copolymers are disclosed in “New Well-DefinedPoly(2,7-fluorene) Derivatives: Photoluminescence and Base Doping,” byM. Ranger, D. Rondeau, M. Leclerc, Macromolecules (1997), Vol. 30, pp.7686-7691 and U.S. Pat. No. 6,204,515 B1, the disclosures of which aretotally incorporated herein by reference.

[0047] Exemplary ladder polymers include for instance ladder-typepolyphenylene and ladder-type polyacene

[0048] where n is from about 5 to about 5,000; R₁, R₂, R₃, and R₄ may bethe same or different from each other and are a side chain comprised of,for example, alkyl derivatives such as alkoxyalkyl and siloxy-subsitutedalkyl, perhaloalkyl such as perfluoroalkyl, polyether such asoligoethylene oxide, polysiloxy derivatives, alkyl oralkoxyalkyl-substituted phenylene, and the like, containing from about 4to 60 atoms. Exemplary ladder polymers are disclosed in “Ladder-typematerials” by U. Scherf, J. Mater. Chem. (1999), Vol. 9, pp. 1853-1864,the disclosure of which is totally incorporated herein by reference.

[0049] The polymer or polymers are completely dissolved or partiallydissolved in a liquid prior to the reducing the solubility of thepolymer or polymers in the liquid. Stirring may be optionally employedto aid the dissolution. Undissolved polymer may be optionally removed byfiltration. The amount of the polymer dissolved in the liquid may rangefor example from about 0.1% to as much as about 50% by weight of thepolymer. In embodiments, the concentration of the polymer in the liquidranges for example from about 0.1% to about 30% by weight, particularlyfrom about 0.2% to about 10% by weight, based on the total weight of theliquid and the polymer. The polymer concentrations in the liquiddescribed above are before reducing the solubility of the polymer in theliquid.

[0050] Heat may be optionally employed to aid the dissolution of thepolymer at an elevated temperature, for a period of time ranging forinstance from about 1 minutes to about 24 hours, particularly from about10 minutes to about 4 hours.

[0051] As used herein, the phrase “elevated temperature” refers to atemperature ranging from above room temperature to the boiling point orhigher of the chosen liquid (at one atmosphere or higher pressure), forexample from about 40 to about 180 degrees C., particularly from about50 to about 120 degrees C.

[0052] The liquid may be a fair to excellent solvent for the polymer atroom temperature or at an elevated temperature. The liquid may be forinstance chloroform, tetrahydrofuran, chlorobenzene, dichlorobenzene,trichlorobenzene, nitrobenzene, toluene, xylene, mesitylene,1,2,3,4-tetrahydronaphthelene, dichloromethane, 1,2-dichloroethanetrichloroethane, 1,1,2,2,-chloroethane, or a mixture thereof.

[0053] Any suitable technique may be used to reduce the solubility ofthe dissolved polymer molecules in the liquid to induce the formation ofstructurally ordered polymer aggregates. For instance, a differentliquid may be added that is less or not capable of dissolving thepolymer compared to the liquid, i.e., the liquid is a better solventthan the different liquid. The different liquid is added in an amountranging from about 1% to about 80% by volume based on the total volumeof the liquid and the different liquid, for a period of time ranging forinstance from about 1 minutes to about 4 hours, particularly from about10 minutes to about 1 hour. Agitation is optionally used during additionof the different liquid. The different liquid may be for instancemethanol, ethanol, isopropanol, hexane, heptane, acetone, and water.Therefore, the resultant composition is comprised of the polymer orpolymers and the combination of the liquid and the different liquidwhich can be the combination of chlorobenzene/hexane,chlorobenzene/heptane, chloroform/methanol, tetrahydrofuran/methanol,and tetrahydrofuran/water.

[0054] In embodiments, the reducing the solubility of the dissolvedpolymer molecules is accomplished by changing the temperature of thecomposition. For example, heat may be used to aid dissolution of thepolymer in the liquid at a temperature and for a time as discussedherein. Then the temperature is lowered from the elevated temperature toroom temperature or by an amount ranging for instance from about 10 toabout 150 degrees C., particularly from about 20 to about 100 degreesC., and wherein the lowered temperature is maintained for a period oftime ranging for example from about 10 minutes to about 10 hours,particularly from about 30 minutes to about 4 hours.

[0055] Any combination of techniques can be used to reduce thesolubility of the dissolved polymer molecules such as using both adifferent liquid and controlling the temperature as described herein.

[0056] Individual polymer molecules tend to come together in aggregateforms when their solubility in the liquid is reduced. These aggregatesmay be amorphous (i.e., disordered) or highly ordered in nature,depending primarily on the polymer structures, the media they are in,and the conditions under which the solubility reduction occurs. Thephrase “structurally ordered polymer aggregates” refers to theaggregation of polymer molecules wherein the spatial orientations orarrangements of the molecules relative to their surrounding neighboringmolecules within the aggregation are orderly in nature. For instance thepolymer molecules may align themselves with their backbones parallel toone another. Changes in molecular ordering of the polymer in acomposition may be monitored by spectroscopic methods, for instance,absorption spectroscopy, optical spectroscopy, NMR, light scattering andX-ray diffractions analysis, and by transmission electron microscopy. Aknown example is regioregular poly(3-alkylthiophene-2,5-diyl)s whichforms π-stacked lamellar structures as a results of its side chainalignment as disclosed in the reference, “Extensive Studies onπ-Stacking of Poly(3-alkylthiophene-2,5-diyl)s andPoly(4-alkylthiazole-2,5-diyl)s by Optical Spectroscopy, NMR Analysis,Light Scattering Analysis and X-ray Crystallography” by T. Yamamoto, etal., J. Am. Chem. Soc. (1998), Vol 120, pp. 2047-2058. The existence ofthe structural order (of the polymer aggregates) is supported by forexample spectroscopy where in an absorption spectrum the absorptionmaxima shifts toward longer wavelengths together with the appearance ofabsorption fine structures (e.g., vibronic splitting).

[0057] Reducing the solubility of the dissolved polymer molecules isaccomplished with or without visually observable precipitation of thepolymer. In embodiments, reducing the solubility of the dissolvedpolymer molecules is accomplished prior to the point that precipitationstarts. In other embodiments, reducing the solubility of the dissolvedpolymer molecules is accomplished beyond the point that precipitationstarts. How much of the different liquid to add or how much should thetemperature drop be to reduce the solubility of the dissolved polymermolecules may be determined for example on a trial and error basis.Precipitated polymer may be optionally filtrated out prior to depositionof the layer of the composition containing the structurally orderedpolymer aggregates.

[0058] Any suitable technique may be used to deposit the layer of thecomposition containing the structurally ordered polymer aggregates. Inembodiments, solution coating may be used. The phrase “solution coating”refers to any liquid composition compatible coating technique such asspin coating, blade coating, rod coating, screen printing, ink jetprinting, stamping and the like.

[0059] During the depositing the layer of the composition, thecomposition may be at any suitable temperature. In embodiments, thedeposition is accomplished at a “lower temperature” below the elevatedtemperature. The “lower temperature” may be a temperature ranging frombelow room temperature to below the elevated temperature such as forexample from about 10 to about 80 degrees C., particularly from about 20to about 40 degrees C., and especially at room temperature. Where thelower temperature is below room temperature, suitable cooling apparatusmay be employed to accomplish this.

[0060] The deposited layer is at least partially dried, especiallycompletely dried, using any suitable technique to remove the liquid (andif used the different liquid). When dried, the polymer aggregatescollapse together and coalesce, resulting in the formation of acontinuous film. This continuous film contains structurally orderedpolymer and thus corresponds to a structurally ordered layer. Dryingtechniques may involve for instance: directing one or more streams ofair (at room temperature or at an elevated temperature) at the layer;“natural” evaporation from the layer (i.e., evaporation at roomtemperature without using an air stream), heating the layer whileoptionally applying a vacuum, or a combination of drying techniques. Inembodiments where heat is employed in the drying technique, the elevatedtemperature may range for instance from about 40 to about 120 degrees C.at normal or reduced pressures, for a period of time ranging forinstance from about 10 minutes to about 24 hours. The dry thickness ofthe layer is for example from about 10 nanometers to about 1 micrometeror for example from about 10 to about 150 nanometers. In embodiments,the structurally ordered layer is the semiconductor layer of anelectronic device such as a thin film transistor.

[0061] In embodiments, the present process may be used whenever there isa need to form a semiconductor layer in an electronic device. The phrase“electronic device” refers to micro- and nano-electronic devices suchas, for example, micro- and nano-sized transistors and diodes.Illustrative transistors include for instance thin film transistors,particularly organic field effect transistors.

[0062] In FIG. 1, there is schematically illustrated a thin filmtransistor (“TFT”) configuration 10 comprised of a substrate 16, incontact therewith a metal contact 18 (gate electrode) and a layer of aninsulating layer 14 on top of which two metal contacts, source electrode20 and drain electrode 22, are deposited. Over and between the metalcontacts 20 and 22 is an organic semiconductor layer 12 as illustratedherein.

[0063]FIG. 2 schematically illustrates another TFT configuration 30comprised of a substrate 36, a gate electrode 38, a source electrode 40and a drain electrode 42, an insulating layer 34, and an organicsemiconductor layer 32.

[0064]FIG. 3 schematically illustrates a further TFT configuration 50comprised of a heavily n-doped silicon wafer 56 which acts as both asubstrate and a gate electrode, a thermally grown silicon oxideinsulating layer 54, and an organic semiconductor layer 52, on top ofwhich are deposited a source electrode 60 and a drain electrode 62.

[0065]FIG. 4 schematically illustrates an additional TFT configuration70 comprised of substrate 76, a gate electrode 78, a source electrode80, a drain electrode 82, an organic semiconductor layer 72, and aninsulating layer 74.

[0066] The composition and formation of the semiconductor layer aredescribed herein.

[0067] The substrate may be composed of for instance silicon, glassplate, plastic film or sheet. For structurally flexible devices, plasticsubstrate, such as for example polyester, polycarbonate, polyimidesheets and the like may be preferred. The thickness of the substrate maybe from amount 10 micrometers to over 10 millimeters with an exemplarythickness being from about 50 to about 100 micrometers, especially for aflexible plastic substrate and from about 1 to about 10 millimeters fora rigid substrate such as glass or silicon.

[0068] The compositions of the gate electrode, the source electrode, andthe drain electrode are now discussed. The gate electrode can be a thinmetal film, a conducting polymer film, a conducting film made fromconducting ink or paste or the substrate itself, for example heavilydoped silicon. Examples of gate electrode materials include but are notrestricted to aluminum, gold, chromium, indium tin oxide, conductingpolymers such as polystyrene sulfonate-dopedpoly(3,4-ethylenedioxythiophene) (PSS-PEDOT), conducting ink/pastecomprised of carbon black/graphite or colloidal silver dispersion inpolymer binders, such as Electrodag available from Acheson ColloidsCompany. The gate electrode layer can be prepared by vacuum evaporation,sputtering of metals or conductive metal oxides, coating from conductingpolymer solutions or conducting inks by spin coating, casting orprinting. The thickness of the gate electrode layer ranges for examplefrom about 10 to about 200 nanometers for metal films and in the rangeof about 1 to about 10 micrometers for polymer conductors. The sourceand drain electrode layers can be fabricated from materials whichprovide a low resistance ohmic contact to the semiconductor layer.Typical materials suitable for use as source and drain electrodesinclude those of the gate electrode materials such as gold, nickel,aluminum, platinum, conducting polymers and conducting inks. Typicalthicknesses of source and drain electrodes are about, for example, fromabout 40 nanometers to about 1 micrometer with the more specificthickness being about 100 to about 400 nanometers.

[0069] The insulating layer generally can be an inorganic material filmor an organic polymer film. Illustrative examples of inorganic materialssuitable as the insulating layer include silicon oxide, silicon nitride,aluminum oxide, barium titanate, barium zirconium titanate and the like;illustrative examples of organic polymers for the insulating layerinclude polyesters, polycarbonates, poly(vinyl phenol), polyimides,polystyrene, poly(methacrylate)s, poly(acrylate)s, epoxy resin and thelike. The thickness of the insulating layer is, for example from about10 nanometers to about 500 nanometers depending on the dielectricconstant of the dielectric material used. An exemplary thickness of theinsulating layer is from about 100 nanometers to about 500 nanometers.The insulating layer may have a conductivity that is for example lessthan about 10⁻¹² S/cm.

[0070] The insulating layer, the gate electrode, the semiconductorlayer, the source electrode, and the drain electrode are formed in anysequence as long as the gate electrode and the semiconductor layer bothcontact the insulating layer, and the source electrode and the drainelectrode both contact the semiconductor layer. The phrase “in anysequence” includes sequential and simultaneous formation. For example,the source electrode and the drain electrode can be formedsimultaneously or sequentially. The composition, fabrication, andoperation of field effect transistors are described in Bao et al., U.S.Pat. No. 6,107,117, the disclosure of which is totally incorporatedherein by reference.

[0071] The TFT devices contain a semiconductor channel with a width Wand length L. The semiconductor channel width may be, for example, fromabout 1 micrometers to about 5 millimeters, with a specific channelwidth being about 5 micrometers to about 1 millimeter. The semiconductorchannel length may be, for example, from about 1 micrometer to about 1millimeter with a more specific channel length being from about 5micrometers to about 100 micrometers.

[0072] The source electrode is grounded and a bias voltage of generally,for example, about 0 volt to about −80 volts is applied to the drainelectrode to collect the charge carriers transported across thesemiconductor channel when a voltage of generally about +20 volts toabout −80 volts is applied to the gate electrode.

[0073] Regarding electrical performance characteristics, a semiconductorlayer comprising the structurally ordered polymer has a carrier mobilitygreater than for example about 10⁻³ cm² Vs (centimeters²/Volt-second)and a conductivity less than for example about 10⁻⁵ S/cm(Siemens/centimeter). The thin film transistors produced by the presentprocess have an on/off ratio greater than for example about 10³ at 20degrees C. The phrase on/off ratio refers to the ratio of thesource-drain current when the transistor is on to the source-draincurrent when the transistor is off.

[0074] In embodiments, the charge transport capability (that is, fordrain current and/or carrier mobility) of an inventive electronic devicemay exceed the charge transport capability of a comparative electronicdevice that is prepared without inducing the increased structural orderin the polymer by an amount ranging for example from about a factor of1.5 to as much as more than an order of magnitude for each of the draincurrent and/or charge carrier mobility

[0075] The invention will now be described in detail with respect tospecific preferred embodiments thereof, it being understood that theseexamples are intended to be illustrative only and the invention is notintended to be limited to the materials, conditions, or processparameters recited herein. All percentages and parts are by weightunless otherwise indicated.

EXAMPLE 1

[0076] a) Synthesis of Polymer

[0077] A polythiophene, poly[2,5-bis(2-thienyl)-3,4-dioctylthiophene](having the structural formula (2) provided earlier in the list ofexemplary polythiophenes), was synthesized using the followingprocedure.

[0078] i) Monomer Synthesis: The monomer2,5-bis(5-bromo-2-thienyl)-3,4-dioctylthiophene for the preparation ofpolythiophene (2) was synthesized as follows:

[0079] 3,4-Dioctylthiophene: 2 M octylmagnesium bromide (100milliliters, 0.2 mol) in anhydrous ethyl ether was added to awell-stirred mixture ofdichloro[1,3-bis(diphenylphosphino)-propane]nickel(II) (0.2 gram) and3,4-dibromothiophene (20.16 grams, 0.0833 mol) in 200 milliliters ofanhydrous ethyl ether in a 500 milliliter round bottom flask cooled withan ice bath under an inert atmosphere. The nickel complex reactedimmediately with the Grignard reagent and the resulting reaction mixturewas allowed to warm up to room temperature. An exothermic reactionstarted within 30 minutes and the ethyl ether began to reflux gently.After stirring for another 2 hours at room temperature, the reactionmixture was refluxed for 6 hours, then cooled in an ice bath, andhydrolyzed with aqueous 2N hydrochloric acid. The organic layer wasseparated and washed successively with water, brine, and again withwater, dried over anhydrous sodium sulfate, and filtered. Afterevaporation of the solvent, the residue was distilled under reducedpressure through Kugelrohr apparatus to provide 21.3 grams of3,4-dioctylthiophene as a colorless liquid.

[0080]¹H NMR (CDCl₃): δ 6.89 (s, 2H), 2.50 (t, J=7.0 Hz, 4H), 1.64-1.58(m, 4H), 1.40-1.28 (m, 20H), 0.89 (t, J=6.5 Hz, 6H); ¹³C NMR (CDCl₃) δ142.1, 119.8, 31.9, 29.6 (2C), 29.5, 29.3, 28.8, 22.7, 14.1.

[0081] 2,5-Dibromo-3,4-dioctylthiophene: N-bromosuccinimide (4.6 grams,25.7 mmol) was added to a well-stirred solution of 3,4-dioctylthiophene(3.6 grams, 11.7 mmol) in a mixture of 30 milliliters of dichloromethaneand 10 milliliters of acetic acid in a 100 milliliter round-bottomedflask. The reaction was monitored by thin layer chromatography and wascomplete in about 35 minutes. The mixture was diluted with 160milliliters of dichloromethane and filtered to remove succinimide. Thefiltrate was washed with aqueous 2N sodium hydroxide solution, and thentwice with water (2×100 milliliters). After drying with anhydrous sodiumsulfate and removal of the solvent, 5.4 grams of2,5-dibromo-3,4-dioctylthiophene as a light yellow liquid.

[0082]¹H NMR (CDCl₃): δ 2.50 (t, J=7.0 Hz, 4H), 1.52-1.28 (m, 24H), 0.89(t, J=6.5 Hz, 6H).

[0083] 2,5-Bis(2-thienyl)-3,4-dioctylthiophene: In a dry box under aninert atmosphere, Pd(PPh₃)₂Cl₂ (0.15 gram, 0.2 mmol) was added to amixture of 2,5-dibromo-3,4-dioctylthiophene (4.2 grams, 9.0 mmol) and2-(tributylstannyl)-thiophene (7.4 grams, 19.8 mmol) in anhydroustetrahydrofuran (50 milliliters) in a 250 milliliter round-bottomedflask. The mixture was then refluxed for 12 hours and the solvent wasremoved by evaporation. The crude product thus obtained was purified byflash chromatography on silica gel using hexane as eluent to give 3.1grams of 2,5-bis(2-thienyl)-3,4-dioctylthiophene.

[0084]¹H NMR (CDCl₃): δ 7.31 (dd, J=3.2, 0.5 Hz, 2H), 7.13 (dd, J=2.2,0.5 Hz, 2H), 7.06 (dd, J=2.2, 4.5 Hz, 2H), 2.68 (dd, J=7.6, 7.6 Hz, 4H),1.59-1.53 (m, 4H), 1.42-1.27 (m, 20H), 0.91 (t, J=6.5 Hz, 6H).

[0085] 2,5-Bis(5-bromo-2-thienyl)-3,4-dioctylthiophene: N-bromosuccinimide (2.8 grams, 15.7 mmol) was added to a well-stirred solutionof 2,5-bis(2-thienyl)-3,4-dioctylthiophene (3.6 grams, 7.6 mmol) ofN,N-dimethylformamide (30 milliliters) in a 100 milliliterround-bottomed flask cooled with an ice-bath. After addition, themixture was allowed to warm up to room temperature slowly. The reactionwas monitored by thin layer chromatography and was stopped after 3 hoursof reaction. The resulting mixture was diluted with hexanes (170milliliters) and washed with three times with 100 milliliters of water.The organic layer was separated, dried with anhydrous sodium sulfate,and vacuum evaporated to provide the crude product, which was purifiedby flash chromatography on silica gel using hexane as eluent to give 2.5grams of 2,5-bis(5-bromo-2-thienyl)-3,4-dioctylthiophene.

[0086]¹H NMR (CDCl₃): δ 7.06 (d, J=3.5 Hz, 2H), 6.86 (d, J=3.5 Hz, 2H),2.62 (dd, J=7.3, 7.3 Hz, 4H), 1.55-1.49 (m, 4H), 1.41-1.28 (m, 20H),0.89 (t, J=6.5 Hz, 6H); ¹³C NMR (CDCl₃) δ 140.6, 137.4, 130.2, 129.3,126.2, 112.0, 31.9, 30.8, 29.8, 29.2 (2C), 28.1, 22.7, 14.2.

[0087] Poly[2,5-bis(2-thienyl)-3,4-dioctylthiophene] (2): A well stirredsuspension of freshly prepared Reike Zn (0.28 gram, 4.29 mmol) inanhydrous tetrahydrofuran (20 milliliters) under an inert atmosphere wasadded dropwise to a solution of2,5-bis(5-bromo-2-thienyl)-3,4-dioctylthiophene (2.46 grams, 3.9 mmol)in anhydrous tetrahydrofuran (10 milliliters), and the mixture waspermitted to react for 45 minutes at room temperature. Subsequently, asuspension of Ni(dppe)Cl₂ (0.021 gram, 0.04 mmol) in anhydroustetrahydrofuran (35 milliliters) was carefully added. The reactionmixture was heated at 60° C. for 3 hours and then poured into 2Nhydrochloric acid solution in methanol. The precipitated polythiopheneproduct was filtered, redissolved in 70 milliliters of hottetrahydrofuran, and precipitated from 2N ammonia solution in methanol.This procedure was repeated twice to remove the acid and oligomers.After drying in vacuo at room temperature, 1.6 grams ofpoly[2,5-bis(2-thienyl)-3,4-dioctylthiophene] (2), M_(w), 41,900, M_(n),11,800 K, Tm, 180° C. resulted.

[0088]¹H NMR (CDCl₃): δ 7.30, 7.13, 7.05, 2.73, 1.59, 1.45, 1.29, 0.89;¹³C NMR (CDCl₃) δ 140.4, 136.7, 135.1, 129.8, 126.4, 123.9, 31.9, 30.7,29.9, 29.3, 28.3, 22.7, 14.2.

[0089] b) Inducing Molecular Self-Organization

[0090] The polymer was first dissolved in chlorobenzene which was a goodsolvent at 2 weight percent level. The solution was filtered through a0.2 μm syringe filter. A poor solvent such as hexane was then addedslowly under shaking or stirring in an amount of about 50% by volume. Noprecipitation of the polymer was observed. The mixture (polymer,chlorobenzene, and hexane) was very stable. It can be stored at roomtemperature for weeks without precipitation. This mixture is ready fordevice fabrication.

[0091] Light absorbance and light wavelength were plotted on a graph todemonstrate solvatochromism of the polymer. A longer wavelength shoulderwas noted in its solution absorption spectrum after hexane addition andwas more pronounced with a red shift as more hexane was added. The redshift is due to the coplanar conformation of the polymer chain withstrong interchain interactions induced by addition of hexane.

[0092] c) TFT Device Fabrication and Characterization

[0093] A bottom-contact thin film transistor structure, as schematicallydescribed by FIG. 1, was chosen as the primary test device configurationin this example. The bottom-contact test device was comprised of aseries of photolithographically pre-patterned transistor dielectriclayers and electrodes with defined channel widths and lengths on a glasssubstrate. The gate electrode on the glass substrate was comprised ofchromium of about 80 nanometers in thickness. The insulating layer was a300 nanometers thick silicon nitride having a capacitance of about 22nF/cm² (nanofarads/square centimeter). On top of said insulating layerwere coated by vacuum deposition the source and drain contacts comprisedof gold of about 100 nanometers in thickness. The polythiophenesemiconductor layer of about 30 nanometers to 100 nanometers inthickness was then deposited by spin coating from the composition(polymer, chlorobenzene, and hexane). The spin coating was accomplishedat a spinning speed of 1,000 rpm for about 35 seconds. The resultingcoated device was dried in vacuo at 80° C. for 20 hours, and was thenready for evaluation.

[0094] The evaluation of field-effect transistor performance wasaccomplished in a black box at ambient conditions using a Keithley 4200SCS semiconductor characterization system. The carrier mobility, μ, wascalculated from the data in the saturated regime (gate voltage,V_(G)<source-drain voltage, V_(SD)) accordingly to equation (1)

I _(SD) =C _(i)μ(W/2 L)(V _(G) −V _(T))²   (1)

[0095] where I_(SD) is the drain current at the saturated regime, W andL are, respectively, the semiconductor channel width and length, C_(i)is the capacitance per unit area of the insulating layer, and V_(G) andV_(T) are, respectively, the gate voltage and threshold voltage. V_(T)of the device was determined from the relationship between the squareroot of I_(SD) at the saturated regime and V_(G) of the device byextrapolating the measured data to I_(SD)=0.

[0096] An important property for the thin film transistor is its currenton/off ratio, which is the ratio of the saturation source-drain currentwhen the gate voltage V_(G) is equal to or greater than the drainvoltage V_(D) to the source-drain current when the gate voltage V_(G) iszero.

[0097] At least five thin film transistors were prepared with dimensionsof W (width)=1,000 μm and L (length)=5 μm.

COMPARATIVE EXAMPLE 1

[0098] At least five thin film transistors were prepared using the sameprocedures of Example 1 except that the polythiophene semiconductorlayer was deposited from pure good solvent, chlorobenzene (that is, nohexane was added).

[0099] The following average properties from at least five transistorsof Example 1 and from at least five transistors of Comparative Example 1were obtained: Mobility Solvent (cm²/V · s) On/off ratio Chlorobenzene(Comp. 0.7 × 10⁻⁴ ˜10³ Ex. 1) Chlorobenzene/Hexane 1.3 × 10⁻³ ˜10⁴(Example 1)

[0100] A dramatic performance enhancement was observed in Example 1.Both the field effect mobility and current on/off ratio were improved byan order of magnitude.

We claim:
 1. A thin film transistor comprising: an insulating layer; agate electrode; a structurally ordered semiconductor layer; a sourceelectrode; and a drain electrode, wherein the insulating layer, the gateelectrode, the semiconductor layer, the source electrode, and the drainelectrode are in any sequence as long as the gate electrode and thesemiconductor layer both contact the insulating layer, and the sourceelectrode and the drain electrode both contact the semiconductor layer,wherein the semiconductor layer is prepared by a process comprising:creating a composition comprising a liquid and a self-organizablepolymer at least partially dissolved in the liquid, resulting indissolved polymer molecules; reducing the solubility of the dissolvedpolymer molecules to induce formation of structurally ordered polymeraggregates in the composition; depositing a layer of the compositionincluding the structurally ordered polymer aggregates; and drying atleast partially the layer resulting in the structurally orderedsemiconductor layer, wherein the structural order of the semiconductorlayer increases the charge transport capability of the semiconductorlayer.
 2. The transistor of claim 1, wherein the reducing the solubilityof the dissolved polymer molecules is accomplished by changing thetemperature of the liquid phase.
 3. The transistor of claim 1, whereinthe reducing the solubility of the dissolved polymer molecules isaccomplished by adding a different liquid that is less capable ofdissolving the polymer than the liquid.
 4. The transistor of claim 1,wherein the different liquid is added in an amount ranging from about 1%to about 80% by volume based on the total volume of the liquid and thedifferent liquid.
 5. The transistor of claim 1, wherein the polymer is aconjugated polymer.
 6. The transistor of claim 1, wherein the polymer isa polythiophene.